This invention relates to polyurethane backings for substrates, more particularly polyurethane backings for carpet which exhibit desirable curing times.
Polyurethane backings are commercially applied to a variety of substrates, the most significant of which are floor and wall coverings such as carpet. One commonly used procedure for preparing such a backing is to position the substrate with its underside up, place a pool or puddle of a liquid urethane forming reaction mixture on to the substrate, gauge the reaction mixture with a doctor blade or similar device to form a film and then cure the gauged film. The liquid urethane reaction mixture is generally applied to the substrate at approximately ambient or slightly elevated temperatures (i.e. 20.degree.-50.degree. C.). However, since the reaction mixture contains catalyst, it often begins to cure before it is gauged.
This curing presents several problems. As the viscosity of the curing reaction mixture rises, it becomes more difficult to gauge the film.
The curing reaction mixture also sticks to the coating apparatus and forms "strings" which disrupt the uniformity of the backing. In addition, this premature curing of the reaction mixture decreases the adhesion of the urethane backing to the substrate, since the higher viscosity thereof reduces the penetration of the reaction mixture into the surface of the substrate.
On the other hand, it is highly desirable from an economics standpoint that the urethane backing cure as quickly as possible once it is gauged and heated to the proper curing temperature. Long cure times delay further processing steps in the production of the backed substrate. Generally, it is desired that the reaction mixture cure to a tack-free state in 10 minutes or less at the desired curing temperature.
Generally, control of the rate of the curing reaction is effected by the use of catalysts. Catalysts such as organotin catalysts and tertiary amine catalysts have been conventionally used in preparing polyurethane carpet backing. Unfortunately, however, the use of these catalysts in conventional amounts generally causes the polyisocyanates and isocyanate-reactive materials to react too rapidly to permit the application and gauging steps to be carried out. Accordingly, it has been attempted to slow the rate of cure simply by reducing the amount of the catalyst used. Although this does sufficiently retard the cure to permit the application and guaging steps, it also substantially increases the time required to cure the mixture to a tack-free state. In addition, using lower levels of these catalysts causes the polyurethane to inadequately adhere to the substrate. Other catalysts, such as mercury and lead catalysts, provide desirable reactivities but are undesirable due to their substantial toxicity.
It would therefore be desirable to provide a catalyst for a polyurethane backing formulation which provides a desired rate of reaction and produces a polyurethane backing which adequately adheres to the substrate.